Optical properties and indirect-to-direct transition of GaP/AlP (001) superlattices

The electronic structure and optical properties of GaP/AlP superlattices are investigated theoretically to clarify the general features of the zone-folding and the band-mixing effects in the superlattice composed of indirect-band-gap semiconductors. The degeneracy of the minimum energies of the conduction band at the X points in the zinc-blende-type bulk materials cannot be lifted by the zone-folding effect alone. The band-mixing effect through the interfaces between two constituent materials plays an important role in determining the overall band lineup throughout the entire Brillouin zone. Thus the electronic structure of the superlattice over the entire Brillouin zone is calculated for the first time in order to determine whether the superlattice is an indirect- or direct-band-gap material. In these calculations, the ${\mathrm{sp}}^3$ $s^{\mathrm{*}}$ tight-binding method is employed which is known to yield a sufficiently accurate conduction band and to give its minimum position correctly. The electronic structure of the superlattice turns out to be quite sensitive to the combination of the well and barrier layer thickness.